This invention relates to a moving target indicating radar system (MTI) and more particularly to an MTI with no blind speeds capable of detecting weak echos from moving targets in the presence of distributed clutter.
MTI systems are well-known in the prior arty but a major problem has been the insensitivity of these systems to targets moving at blind speeds. A target moving at a blind speed will move toward or away from the MTI system an integral number of half wavelengths of the transmitted radio frequency beam from the MTI during the interval between transmitted pulses.
One method of eliminating blind speeds, disclosed in U.S. Pat. No. 3,225,349 by Thor, utilized expanded transmitted signals that are linearly frequency modulated. A first pulse with a linearly increasing frequency (the upswept pulse) is followed by a second pulse with a linearly decreasing frequency (the downswept pulse). The upswept and downswept pulse are generated by applying a drive pulse to an appropriate filter.
The echos of the upswept and downswept signals from targets are received and channeled through matched filters. These matched filters compress the upswept and downswept pulses so that accurate timing measurements are possible.
The received pulses are coherently or non-coherently subtracted so that echos from non-moving targets are cancelled. However, echos from moving targets have compressed pulses that are displaced slightly in time from a compressed pulse from a non-moving target due to range doppler coupling. The sign of this time displacement is dependent upon whether the echo is the reflection of an upswept or a downswept pulse.
Thus, due to range doppler coupling, the echos from a moving target of successive upswept and downswept signals will not cancel in the subtractor. Therefore, only moving targets will be detected.
The sensitivity of the above-described MTI may be increased by utilizing polyphase coded expanded output pulses since the corresponding compressed pulses have lower sidelobes than linear pulses. However, one problem with using these polyphase coded pulses is that the autocorrelation sidelobes of the pulses have complex values. As described below, the sidelobes of the echos from distributed clutter will not cancel in the MTI subtractor if the sidelobes have unequal complex values. Therefore, weak echos reflected from moving targets may be masked by these uncancelled sidelobes from clutter thereby decreasing the sensitivity of the MTI radar system.